This invention relates to luminescent materials and methods of making luminescent materials. More particularly, it is concerned with a method of making a substantially stoichiometric zinc silicate phosphor having improved brightness and persistence.
Manganese activated zinc silicate is a well known green emitting cathodoluminescent phosphor identified commercially as type P-1. When manganese is incorporated into zinc silicate, the resulting material is often off-white or dark in appearance. A white-bodied manganese activated zinc silicate phosphor can be produced by employing excess silica over the amount stoichiometrically required during the formulation of these phosphors as taught by U.S. Pat. No. 2,245,414 to Roberts, or by incorporating small amounts of magnesium ion into the zinc orthosilicate lattice as disclosed in U.S. Pat. No. 3,416,019 to Kaduk.
The P-1 phosphors, however, are generally characterized by rapid fluoroescence decay and can exhibit the undesirable visual effect of flicker when used in such applications as cathode ray tubes operated at low refresh rates. To improve the persistence of manganese activated zinc silicate phosphors, small amounts of arsenic are added as taught by U.S. Pat. No. 2,554,999 to Merrill et al. Arsenic-containing manganese activated zinc silicate phosphors are identified commercially as type P-39.
While persistance in type P-39 phosphors generally increases with increasing arsenic content, it does so at the expense of brightness. Often, small changes in arsenic concentration will produce large changes in phosphor brightness. Thus, in producing these phosphors, an attempt is made to carefully control the arsenic concentration to effect a trade-off between enhanced persistence on the one hand and diminished brightness on the other.
However, a rigorous control of the arsenic concentration is often difficult, owing to the tendency of arsenic compounds to volatilize from the phosphor formulation during the repeated high temperature firings often required for the formation of zinc silicate. To facilitate the formation of zinc silicate, it has been the practice to employ alkali metal or alkaline earth metal salts as fluxes during firing as taught by U.S. Pat. No. 2,247,192 to Fonda. The indiscriminate use of such fluxes is not desirable, however, since fluxes such as lithium chloride can completely quench the effect of extended fluorescence conferred upon P-39 phosphors by the incorporation of arsenic [see for example, Froelich and Fonda in J. Phys. Chem., 46:878-885 (1942)].
In U.S. Pat. No. 4,315,190 to Peters et al., assigned to the assignee of the present invention, there are described type P-39 zinc silicate phosphors modified by the addition of small amounts of alkali metal salts to enhance brightness and persistence.